Thermocouple ammeter



Nov. 23, 1937.

MILLER 2100,26

THERMOCOUPLE AMMETER Filed Sept. 11, 1936 3)WC rytM Patentedi Nov. 23, 1937 man STATES PATENT OFFICE 2,10026! THERMOOOUPLE All/METER John H. Mller, Short Bills, N. .ll. assignor to Weston Electrical Instrument Corporation, Newark, N. J., a corporation oi' New Jersey Application September 11, 1936, Sex-1a] N0. 100,372

8 Claims.

The method of operation of the thermocouple axmneter ls, a.s is well known, the heatlng o1 e. reslstance wlre by the current to bemeasured, and the measurement of the temperature of the reslstance wlre by a thermocouple e.nd a. millivoltmeter.

'I'he patents to William N. Goodwin, Jr. Nos. 1,407147 wnd 1,456591 describe methods of compensatlng ior thermally-produced errors, anti thermocouple a.mmeters constructed in accordance wlth the design requirements stated in those patents have been in use in the radio lndustry for many years. This compensated type cf therxnocouple ammeter ls themost satisfactory instrument for the measuriament of hlgh frequency currents but the progressive extenslon of the radio development work in the field of "ultraxhlgh, frequencles has shown that there is a. frequency limit beyond which the indlcated. current flow ls substantially higher than the actual eurrent flow. Errors of the order of up to 100% have beten observed when 5-an1pere thermocou ple ammeters cf the prlor designs were used a1; 100 mega.cycles.

A conslderation of the progressive lncrease in the error with frequency, and of the temperature-measurement method of operation of the thermocouple ammeter lndlcates that this error als due to shn efiect or apparent lncrease in the resistance of the heater element as the frequency cf the hea.tlng Current is lncreased. It has been proposed to coTrect for thls error by compensat- Ing networks, but such compensatlon is difilcult in view 02 the low reslstance 013 the heater element, whlch ls, for exa mple, only about 0.04 011m in a 5-ampere Instrument. Furthermore compensatlon ls usually obtalnable only in the region of one frequency. and large errors exist a1: other frequencles.

' Objects of the present inventlon are to pro vide thermocouple ammeters, and thermocouples for assoclatlon with milllvoltmeters as current measurlng devices, that will indicate current flow in the ultra-hlgh frequency range with high accuracy.

An object ls to provide a. thermocouple ammeter of novel construct lon in whlch the temperature attalned by the thermocouple heater element ls approxlmately the same, for a. glven current flow, at high frequencies and a1; ultrahlgh Irequencles. More speclficallm objects of thls inventlon are to provide thermbcouples and thermocouple ammeters in which the heater element is of tubular form and has an elfective reslstance tliat is approximately independent 01 the frequenc'y of the current to be measured.

These and other objects and advantages of the invention will be apparent from the follow- Ing speclfication when taken with the accompanyl ng drawlng in whlch:

Fig. 1 is a plan view of a thermocouple am- 1 meter embodying the invention, the milllvoltmeter being shown dlagrammatically;

Flg. 2 is a. side elevatlon of the thermal converter unit o1 Fig. 1; ancl Flg. 3 ls a perspectlve view, on a greatly en 1 larged scale, of the heater element and thermocouple junctlon.

In the drawlng, the reference numera.l I identifies a millivoltmeter that ls connected to the compensating strips 2 or cold termlnals of a 2 thermocouple conslsting of wires 3, of dissimlla.r metals. 'I'he junctlon oi' the thermocouple is thermally connected to the center of a heauer, as will be described later, and the ends of the thermocouple wires are secured to the centers of 2 the respectlve compensating strips by solder 5. The ends of the compensating strips are thermally connected to but electrica.lly insulated from relatlvely massive heater terminals 6 by thin sheets of mica l, and these parts are held in as- 3 sembled positlon by pleces of insulating material 8 that a.re clamped to the terminals 6 by screws 9. Lugs or inserted heavy wires I0 extend from the opposed s'urfaces of the terminals 6 and are bent upwardly lnto proximity to sup- 35 port a short heater element.

The constructlon so far descrlbed will be recognized as a thermocouple ammeter of the compensated type described in the Goodwln patents. Accordlng to prlor practlce, the heater element 4( for a 5-ampere ammeter may have been, for example, a platinum lrldium wire of 11 mil diameter (0.011 inch) and 0.2 lnch long. The resista.nce of such a heater element at 100 megacycles is about 2.6 tlmes its low frequency re- 45 sistance, and the net effect of this resistance increase on the Instrument scale ls proportional to the square root of the reslstance inrease so that the lndlcated current values at 100 megacycles are about 1.6 times the true value of the current.

y In accordance with the present invention, the heater H is not a wire er strip bul. is a thin walled tubular element which may be formed by benc ling a thin metal sheet into cylindrical shape. As shown in Fig. 3, the longitudinal edges of the .eta1 sheet are ciosely adjacent, but need not a overlapped or secured to euch other, anti this int is preferably a.t the bottom of the assembled 1ermal converter. 'I'he thermocouple junction, r preferably a. short extensin 3' 013 one thermouple wire beyond the junction, is thermally mnected to the Center cf the tubular heater. his connection may be made by sliding the tuular hea.ter on a. Support-Ing mandrel, and spot e1ding the extension 3' to the bester.

'I'he hea.ter tube l l Is then mounted on the terdnal assembly by pla.cing the tube I l on the suporting wires I0 and securing It in plane by so1der 2. 'I'he effective length of the hea.ter Is deterxined by the spacing ot the supporting wires but ne assemblyis facilitated by forming a. relativeiy ng tube am]. cutting ofl 01' flattening the tube eyond the Supports I0. The requirements Ior btaining thermal compensations determine thezngth cf the tubular heater and In general this zngth may be of the order of 0.20 to 0.25 Inch. 'he skin effect or the rate at which the resistance f the heater l l increases with Irequency depends pon the wall thickness, und I ha.ve found tha.t he resistance may be maintained approximately onstant for frequenci'es up to abput 100 megaycles by forming the tubes from 1 mil platinum esistance material. A tube diameter 02 28 mils s satisfactory for a. -ampere Instrument, a.nd a.rger diameter tubes will be used. for bisher urrent ranges if the sa.me wa1l thickness is used or a. serIes 01 Instruments of difierent current anges. A

The advantages of the Invention will be evilent from a compaxison o1 the performance 01 l, typical 5-ampere Instrument, as described nbove, with th.at 0I the previously known thermo- :oup1e ammeters. One Instrument with a. tubuar heater element gave a. reading I!or 3 amperes |.t 80 megacycles that was too high by about 5%, while the corresponding readings of Instrunents of prior designs were too high by as much tS from 45 to 70%. 'I'he corre'ction factor Ifor the aew Instrument was 0.94 while the correction :'actors for the prior Instruments ranged trom 1bout 0.69 to 0.59.

It is obvious that any Instrument o! stableon- ;truction may be calibrated to read accurately at some one frequency but the present Invention :ontemplates Instruments tha.t have s. high ac- :uracy over an extended range 01 high and ultrahigh frequencies. Errors of the order of up to 6 a.nd 7% are usua.lly permIssible in radio work but greater accuracy may be had by employing a corre'ction factor. 'I'he tubula.r type of bester element Is particularly adva.ntageous from this standpoint, a.s the sa.me correction factor is theoretica.lly appropriate, at any given frequency. Eor a series of Instruments o1 diflerent current range havlng bester tubes 0I 1 m11 weil thickness Illustrated und described, or to constructions which aflord the maximum ac'curacy a.t any particular frequency or Irequency range as the principies herein stated may be applied in the design of other physical constructions to decrease the errors due to frequency variant resistance of the hea.ter elements of thermocouple arnmeters.

I claim:

1. In an ammeter for use at high frequencies, the combination with a. millivoltmeter and a thermocouple, of a. heater element In hea.t trans- Ier relationship to seid thermocouple, seid heater element being' tubular und having a wa.ll thicknass o1 the order of not more than 0.004 inch.

2. A thermal ammeter as clalmed in claim 1,-

wherein the tubula.r heater element has a. diamet er which imiaarts to the heater element at maximum current fiow therethrough the electrical charwcteristic of an approzdnmtely constant resistance over a range o! frequencies extending trom one megacycle to an order substantially higher.

3. An ammeter as claimed in claim 1, wherein seid heater element has a. wall thickness of the order oI" 0.001 Inch. L

4. An 'ammeter as c1a.imed In claim 1, wherein said tubula.r heater element comprises a th1n sheet of metal bent to cylindrical form.

5. A thermal converter for use with a millivoltmetezr In the measurement of high Irequency currents, said therma.l Gouverter comprlsin s. thin walled tubuia.r hea.ter element, and a thermocouple exterior to and having Its Junetion positioned on the longitudinal center of said heater element.

6. A thermal converter as claimed incla.im 5, wherein seid tubuiar bester element ha.s a. wall thickness o1 the order of 0.001 inch.

7. A thermal converter a.s cla.imed In claim 5,

wherein seid tubular bester element comprises 

